1. Field of the invention
The present invention relates to a method for forming a photo diode on a semiconductor wafer, and more particularly, to a method for forming a photo diode and a CMOS transistor simultaneously.
2. Description of the prior art
The photo diode is a semiconductor device comprising a photo-conductivity cell and a junction diode, and is commonly applied to manufacture photoelectric products, such as cameras and photo sensors of scanners. The light current of the photo diode represents signals, the dark current represents noises, and the photo diode processes signal data by using the intensity of the signal to noise ratio. In the semiconductor industry, it is often to increase the light current of the photo diode so as to increase the signal to noise ratio, and hence to enhance the contrast of the signal. As a result, the sensitivity of the photo diode is enhanced and the quality of the photo diode is improved.
Please refer to FIG. 1, FIG. 1 is a schematic diagram of a photo diode 10 according to the prior art. A prior art photo diode 10 comprises a P-type well 16 positioned in a predetermined region on the semiconductor substrate 14, and a photo sensing area 12 formed on the surface of the P-type well 16. The semiconductor wafer further comprises a field oxide 18 surrounding the photo sensing area 12 on the semiconductor substrate 14 to form an electrical isolation to prevent short circuits.
The method of forming the photo diode 10 according to the prior art uses high dosage arsenic (As) atoms as the major dopant to proceed an ion implantation process. The energy and the dosage for the ion implantation process is 80 KeV and 10.sup.5 cm.sup.-2, respectively. This ion implantation process forms a N-type doped area 13 on the surface of the P-type well 16. A depletion region 15 for detecting the light current is formed along the PN junction between the doped area 13 and the adjacent P-type well 16. In FIG. 1, the area labelled by slanting lines illustrates the depletion region 15.
In the formation of the photo diode 10 according to the prior art, As atoms at high dosage for forming the source and the drain of the MOS transistor are used as the major dopant to simultaneously form the doped area 13 on the surface of the P-type well 16. Therefore, the crystal structure on the surface of the photo sensing area 12 may be damaged by high dosage ions during the ion implantation. As a result, the PN junction of the photo diode 10 generates more dark current, resulting in higher noises of the photo diode 10. Also, the width of the depletion region 15 is narrower, resulting in a smaller real active region of the sensing area 12. Therefore, when light irradiates the photo diode 10, the light current sensed by the depletion region 15 is reduced and the signal to noise ratio is lowered. And hence, the sensitivity of the photo diode 10 is reduced.
In addition, using a higher implantation energy results in a deeper PN junction. When the photo diode 10 is irradiated by short wavelength lights, the blue light for instance, the light current induced by the PN junction of the photo diode 10 is smaller because the short wavelength light has a shallow penetration depth in the silicon wafer. Therefore, the sensitivity of the photo diode 10 for detecting the short wavelength light is reduced.